Computed tomography (CT) is undeniably an important technology in modern medicine. CT is used throughout screening, treatment, and follow-up examinations; it is not uncommon for a patient to receive ten CT scans during this time.
In light of the increasingly frequent use of CT for cancer imaging, high radiation dose from CT scans are concerning. There is strong scientific evidence that the elevated dose exposures as found in CT are leading to a significantly increased risk of cancer, especially in young patients. The National Academy of Sciences BEIR V (Biological Effects of Ionizing Radiations) committee and the ICRP (International Commission on Radiological Protection) have reported that for a single acute radiation exposure in children the lifetime attributable cancer mortality risk is as high as 14% per Gy. This risk goes down with age, but is still in the 2-8% range in the 30-50 age group. Importantly, recent reports on the biological effects of radiation reaffirm the utility of the linear no-threshold model of radiation risk for solid cancers. Brenner et al. have shown that the average dose of a single CT scan leads to about 5 cancer deaths in 10000 patients depending on the age of the patient and type of exam performed.
The magnitude of the problem is worsening due to the large number of CT scans performed every year (62 million in the United States in 2006). While CT accounts for only 17% of imaging procedures using radiation, it delivers 49% of the overall dose. The NCI (National Cancer Institute) website states: “CT is the largest contributor to medical exposure to the U.S. population.”
CT is currently being investigated as a screening tool for early, asymptomatic cancer detection. A recent publication in the New England Journal of Medicine by the International Early Lung Cancer Action Program Investigators concludes: “Annual spiral CT screening can detect lung cancer that is curable.” The NCI is currently conducting the National Lung Cancer Screening Trial (NLST) with the objective to qualify CT and other X-ray modalities for lung-cancer screening. One potential outcome of this trial is the recommendation to screen a large population for lung cancer with CT, thus increasing the radiation exposure to a large group of asymptomatic people. In fact, a recent press release reports that NLST “found 20 percent fewer lung cancer deaths among trial participants screened with low-dose helical CT.” Additionally, NCI is funding an R01 grant to develop an iterative-reconstruction algorithm for dose reduction in CT lung-cancer screening. Thus, the prospect of CT use in cancer screening warrants investigating ways to make CT more dose efficient.
Colorectal cancer, the fourth leading cause of cancer deaths worldwide, is largely preventable with appropriate screening. Optical colonoscopy is the method of choice. However, the nature of the exam has led to low compliance with the recommended screening intervals. Virtual colonoscopy with CT is emerging as an alternative that potentially could lead to much higher compliance rates. For example, for President Obama's yearly health checkup, virtual colonoscopy was chosen over optical colonoscopy. Again, if virtual colonoscopy gains traction as a screening tool, a large number of asymptomatic people will receive regular CT scans with high radiation exposure.
Another area of immense concern is pediatric cancer imaging. A recent study by Robbins evaluates the treatment protocols by the Children's Oncology Group that are typically used in the United States. The study shows that throughout diagnosis, treatment, and follow-up periods for childhood cancers such as neuroblastoma, Wilms tumor, Ewing sarcoma and lymphoblastic lymphoma, the radiation dose from imaging studies ranges between 109 and 152 mSv. The radiation dose is mostly from CT with the evaluated cases involving more than 15 CT scans each. Based on the BEIR V and ICRP reports, the lifetime risk of these children developing a fatal cancer is in the 1-2% range. While pediatric cancer is a small fraction in the overall cancer problem, children are a particularly vulnerable group.
These examples highlight the broader problem of high radiation exposure in cancer imaging with CT. Therefore, it is desirable to reduce radiation dose in CT to continue the impressive success of CT in fighting cancer and at the same time reduce the risk of causing cancer with the very same modality.
Furthermore, modern computed tomography (CT) scanners have the goal of covering a large volume of the patient in a single rotation at very fast rotation speeds. This objective is driven by demands of cardiac CT to cover the entire organ in less than a heartbeat. Impressive results have been achieved with the current generation of CT scanners. However, the downside of this development is the increased dose to the patient, the increase in scatter, and the degradation of image quality in the outer slices due to cone beam artifacts. In particular, the increased dose in medical imaging has come under scrutiny, with several published studies documenting the elevated risk of cancer resulting from the radiation involved in medical imaging.
CT manufacturers are exploring a variety of methods to reduce this dose while maintaining image quality. However, these improvements are expected to be minor compared to that which may be gained by an alternative CT system concept, inverse-geometry CT (IGCT). Conventional point source CT utilizes a single focal spot X-ray source and a large-area detector, whereas IGCT utilizes a large-area, multi-focal spot X-ray source and a small-area detector. IGCT offers higher dose efficiency and faster acquisition times than state-of-the-art conventional point source CT systems. Thus, IGCT has the potential to overcome disadvantages with conventional point source CT and significantly out-perform conventional point source CT scanners.
However, IGCT as currently realized in prototypes faces difficulties in implementation due to a large source array to be rotated at high speeds and significant challenges from high power and cooling requirements of the source.
What is needed is a CT imaging system capable of producing rapid high quality images. Furthermore, the CT imaging system should provide low radiation imaging.